1. Field of the Invention
This invention relates generally to flash memory devices and, more particularly, to a method of rotating usage of memory partitions to extend the life of the flash memory device.
2. Related Art
Flash memory is a single transistor cell which is programmable through hot electron injection or source injection and erasable through Fowler-Nordheim tunneling. The programming and erasing of such a memory cell requires current to pass through the dielectric surrounding a floating gate electrode. Because of this, such types of memory have a finite number of erase-write cycles. Eventually, the dielectric will fail. Manufacturers of flash cell devices specify the limit for the number of erase-write cycles as between 10,000 and 100,000. Some flash technologies have over-erase problems, therefore they need to be programmed before erasing to improve uniformity. They also have to be verified for successful erase. Accordingly, unlike rotating magnetic media, a flash memory device does not have an indefinite lifetime.
Flash memory devices have many advantages for a large number of applications. These advantages include their non-volatility, speed, ease of erasure and reprogramming, small physical size and related factors. There are no mechanical moving parts and as a result such systems are not subject to failures of the type most often encountered with hard disk storage systems.
Flash memory devices have many characteristics that are different from other memory devices. One major difference is that a block containing existing data in flash memory devices cannot be overwritten with new data. Existing data must be completely erased (also referred to as “cleaned”) from a block before data can be written into memory locations again. For the reasons noted above, blocks can be erased only a limited number of times before the flash memory device becomes unusable.
Additionally, most block erasures stall other operations from occurring such as read and write operations to the flash memory device. Most flash memory systems attempt, therefore, to minimize erasures to specific times, such as at initialization or powering-off of a computer system, or at periodically scheduled times.